No other field is philosophy. Philosophy, in some respects, is special - at least according to Socrates as relayed by Plato. As Socrates says, each science (or even activity) has its object, but what of the science which has science as its object? The trouble with many future reformulations of key ideas is that they can miss the intricacies and highly abstract arguments of the original texts. For that reason, it may be more appropriate to read Newton than it is to read Einstein, because Newton used much more natural language than equations. I would very heavily disagree with the idea that introductory textbooks necessarily have better contents and presentation than original texts, and any serious philosopher will tell you that too.
>If you are telling me that in the twenty centuries, no one has taken what Pythagoras/Plato said and has rewritten it in a better way, with modern exposition and examples and graphics and what not, I fail to believe you.
What do you mean by "a better way"? Is Plato really so difficult or obtuse or obscure that one needs to express Socrates' ideas in "a better way"? The text is perfectly readable to a modern audience, which is precisely why starting with Plato's actual dialogues is the hallmark of any good university philosophy programme.
>Because that would mean philosophy as a discipline is not improving by building upon the works of those past
No; you have it backwards. Many philosophers have agreed with or disagreed with Plato, but the point of a text on Plato's ideas should represent what he wanted to represent in those ideas, not say "well, that's what Plato said, here's my idea now!". Philosophy (according to some accounts, at least) does progress, but it's dubious that rewriting and "simplifying" texts is the hallmark of progress.
You end with the assumption that books written since 2000 are necessarily better than Plato. I fail to see why this is the case. If you want an overview of philosophy since Plato, then by all means, only the most up-to-date book will do. If you want to know Plato's (or Socrates') ideas yourself, there is no better source than Plato himself.
Philosophy explanations and short introductions (Peter Singer's one on Marx comes to mind) are riddled with errors, and for any such text you will find at least ten philosophers ready to lambast an "introductory" interpretation because it misses out on the intricacies of that thought. I'm not joking. Look at any intro book on Hegel, Kant, Marx, Pythagoras. Then look at the reviews published in philosophy journals.
"Progress" in philosophy does not and cannot mean shortening, rewriting and "simplifying" ideas.